The present invention relates to electrically-conductive polymeric compositions, and more particularly to form-in-place EMI shielding gaskets and methods for forming such gaskets.
EMI shielding gaskets are used on electroninc equipment to provide protection against interference from electromagnetic energy, includingradio frequency interference (RFI) and more broadly all bands of interference commonly called electromagnetic interference (EMI). The shielding gasket has conductive filler or conductive element, be it a wire mesh, conductive filler or conductive plating, coating or fabric which prevents external EMI from interfering with an electronic device and/or protects other adjacent electronic devices from EMI emitted by an electronic device.
Typically, EMI gaskets are prepared in one of three configurations: linear, die cut or molded. By linear, it is meant as an extrusion, molding, etc. of a defined, straight length. By die cut, it is meant that a gasket configuration is formed from an electronically conductive elastomer sheet material which is cut by a die to the desired shape, such as round, square, etc. By molded, it is meant that the gasket configuration is formed by placing uncured elastomer which may contain conductive filler or a conductive mesh, into a specifically designed mold which then is subjected to compression (pressure) and then cured to cause the elastomer to assume the desired gasket configuration.
All three methods have disadvantages especially when used to form complex multidirectional or multiaxial gaskets, such as may occur in devices with a number of compartments that each need to be shielded, from each other as well as the external environment. Moreover, the problems are even more critical on smaller devices, such as cellular phones, notebook computers and other hand held devices, where the diameter of the gasket becomes very small and the ability to manufacture and attach such gaskets securely becomes very difficult and labor intensive.
Using linear gasketing material to form complex multiaxis/multidirectional gaskets (e.g. either x and y or in the x, y and z planes), is difficult, time consuming and costly. Each gasket portion must be hand cut and bonded to the adjacent portions of other linear gaskets and then bonded or secured in position upon the substrate.
Die cutting of conductive sheet stock will work in many instances especially in two plane (e.g. flat; x, y) applications, provided that each portion of the gasket is wide enough and/or thick enough to be self supportive. Die cutting parts however results in significant waste of the sheet stock because the material is typically a cross-linked resin such as silicone or polyurethane. This is not acceptable as it drives up the cost of such parts unacceptably. Further as die cutting is a rough process, the sheet stock needs to be fairly stiff and self supportive which is opposite that desired by the gasket user (i.e. soft and flexible).
Molding is slow and again generates scrap in the form of flash which must be removed. Furthermore, each gasket design must use a specifically designed mold, making the process expensive for all but large volume stock items.
A form-in-place EMI gasket and system for forming complex multiaxis/multidirectional EMI gaskets which generates a minimum of scrap, which forms the gasket in place and requires no special tooling is desired. The present invention provides such a system.
The present invention relates to an EMI shielding material that can be used to form a form-in-place EMI gasket, and a system for forming such a gasket using a table and/or dispenser capable of moving in multiaxial directions relative to each other and a substrate to be gasketed. The invention also relates to a process of providing a conductive elastomer, forming it in place along a desired gasket configuration so as to create a form-in-place EMI gasket.
According to one embodiment the invention provides EMI shielding material defined in part by a polymeric thermal addition cure system including an electrically-conductive filler. According to this and other embodiments, the polymeric system can include an elastomer base. The material desirably has bulk resistivity of less that about 0.050 ohm cm, and is readily extrudable. Upon exposure to a temperature of at least 85xc2x0 C. for a period of time of at least 30 minutes the material becomes essentially thermoset. The polymeric thermal addition cure system is defined, according to one aspect, by a first species having a first functional group, a second species having a second functional group that is reactive with the first functional group in the presence of a catalyst and heat, and a catalyst having catalytic action that in the presence of heat catalyzes a reaction between the first and second functional groups.
According to one embodiment of the invention the electrically-conductive filler inhibits the catalyst to adversely affect the catalytic activity of the catalyst, yet the catalyst is present in an amount sufficient to retain desired catalytic activity. Methods and arrangements for retaining the catalyst in an amount sufficient to retain catalytic activity are provided, and according to one the shielding material is packaged as a kit. The kit includes a first container containing a first pre-mixed component including the first species, the electrically-conductive filler, and the catalyst, a second container containing a second pre-mixed component including the first species, the second species, and the electrically-conductive filler. The kit includes as well a third container containing the catalyst. By this arrangement, the third container is used to replenish catalyst that is inhibited by the electrically-conductive filler. By xe2x80x9cinhibitedxe2x80x9d is meant chemically or physically affected in a way that catalytic activity is reduced. According to one embodiment of the invention an FMI shielding material includes a siloxane polymer including vinyl functional groups, a siloxane cross-linker including reactive hydrides, a silver-containing electrically-conductive filler, and a platinum catalyst. The silver-containing electrically-conductive filler inhibits the platinum catalyst, thus methods and arrangements of the invention provide for replenishment of the catalyst to insure that sufficient catalytic action can be provided.
According to another embodiment of the invention a method of making an EMI shielding gasket is described. The method includes extruding onto a substrate a free-form polymeric thermal addition cure system, which includes an electrically-conductive filler and which preferably has a viscosity of from about 100,000 to about 10,000,000 centipoise, more preferably from about 1,000,000 to about 4,000,000 centipoise. The system then is heated at a temperature and for a period of time sufficient to cure the system, thereby forming a gasket having bulk resistivity of less that about 0.050 ohm cm. The gasket can be formed without applying pressure, for example with a mold, to the to the extrudate. Prior to the extruding step, according to one aspect, the polymeric thermal addition cure system is prepared by forming a mixture of a first species having a first functional group, a second species having a second functional group that is reactive with the first functional group in the presence of a catalyst and heat, a catalyst that catalyzes a reaction between the first and second functional groups, and the electrically-conductive filler.
According to one aspect of the method, the polymeric addition cure system is prepared by mixing together a first pre-mixed component and a second pre-mixed component. The first pre-mixed component includes the first species, the electrically-conductive filler, and the catalyst. The second pre-mixed component includes the first species, the second species, and the electrically-conductive filler. Prior to mixing together the first and second components, additional catalyst can be added to the first pre-mixed component. This arrangement can be used when the catalyst has catalytic action that in the presence of heat catalyzes a reaction between the first and second functional groups, and the electrically-conductive filler inhibits the catalyst to adversely affect the catalytic activity of the catalyst.
The invention also provides an EMI shielded substrate. The shielded substrate includes a first electrically conductive substrate, a second electrically conductive substrate adjacent to the first substrate, and a formed-in-place electrically conductive gasket formed on and bonded to a predetermined portion of the first substrate so as to provide an electrical connection and FMI shielding between the first and second substrates. According to one aspect the first substrate is an enclosure and the second substrate is a cover for the enclosure. The first and second substrates can be formed of an electrically conductive material such as metal, metal plated plastic, metal/plastic laminate and composite, coated plastisol, or any combination of these. The gasket can be formed of an elastomer resin and rendered electrically conductive by the incorporation of one or more conductive fillers into the resin. The gasket can include more than one layer, with at least an outer layer being electrically conductive.
According to one embodiment the EMI shielded substrate includes a substrate having an electrically conductive surface, a cover for the substrate, the cover having an electrically conductive surface which corresponds to and is in register with the conductive surface of the substrate, and a formed in place electrically conductive gasket formed on and bonded to a predetermined portion of the conductive surface of the substrate or cover so as to provide an electrical connection and EMI shielding between the substrate and cover upon the mating of the cover to the substrate.
According to another embodiment of the invention a form-in-place EMI gasket is provided that includes a composition formed of a silicone resin, one or more conductive fillers, and a curing agent for the resin. The composition, when mixed and applied to a substrate, will create a form stable, form-in-place gasket capable of providing EMI shielding of from about 10 dB to about 120 dB over a frequency range from 10 MHz to 12 GHz.
According to another embodiment the invention provides an EMI gasket that is made of a composition formed of a first component which is a primary polymer having end groups that are capable of chemically reacting with each other in the presence of moisture to form a derivative polymer having a longer average chain length than said primary polymer, a second component which is a noncross-linked elastomer that is not substantially chemically reactive with itself or with said first component, and a third component which is one or more electrically conductive fillers. When the first, second, and third components are intimately mixed, the composition, when maintained in the absence of moisture and other active hydrogen donor materials, is readily extrudable and otherwise conventionally moldable thermoplastic composition. Upon exposure to moisture, however, the composition becomes essentially thermoset.
According to another embodiment, the invention provides a system for forming EMI shielded enclosures. The system includes a support platform, a compound applicator nozzle located above and in register with the platform, a supply of electrically conductive compound connected to the applicator nozzle, and a drive mechanism for moving the nozzle or platform relative to each other in one or more directions of travel. A curing chamber for curing the compound, after application, can be provided.
electrical bridge or continuity between the two substrates and thereby prevent or reduce the potential for EMI.